Thermoplastic polyester resins such as polyethylene terephthalate and polybutylene terephthalate are excellent in mechanical characteristics, heat resistance, chemical resistance and dimensional stability and are widely used in the fields of injection-molded articles, fibers and films. However, it is difficult that the thermoplastic polyester resins during melting are brought into such viscoelastic properties that foams can be obtained. Hence, a blowing agent is easily released during foam extrusion and it is difficult to obtain good foams wherein fine closed cells are uniformly formed. To solve this problem, there was proposed a method wherein diglycidyl esters are incorporated in aromatic polyesters in the foam extrusion of the aromatic polyesters [see, JP-B-61-48409 (the term "JP-B" as used herein means an examined Japanese patent publication)]. In the foam extrusion of thermoplastic polyesters, there was proposed a method wherein polyfunctional diglycidyl esters and poly-functional carboxylic acid anhydrides are incorporated in thermoplastic polyesters to improve the melt viscosity of the thermoplastic polyesters [see, JP-A-59-210955 (the term "JP-A" as used herein means an "unexamined published Japanese patent application )].
The present inventors have been engaged in the production and study of extruded foams for many years and have had experience in that when the foam extrusion comprising a thermoplastic polyester in admixture with diglycidyl ester compounds is subjected to continuous long run, the resulting foam becomes discolored. The present inventors have continued to make the study and found that when pyromellitic dianhydride is mixed with a thermoplastic polyester without using any diglycidyl ester compound, the resulting foam is not colored and burned even when foam extrusion is continuously conducted over a long period of time.
The present inventors have further continued making the study and found that when compounds having two or more acid anhydride groups per molecule such as pyromellitic dianhydride and compounds of Group I, II or III metals of the Periodic Table are added to thermoplastic polyester resins, the viscoelasticity of the molten materials is improved and at the same time, there can be obtained foams having high tensile elongation and more finer cells.
JP-A-59-135237 (U.S. Pat. Nos. 4,462,947 and 4,466,943, European Patent 0115162A) discloses that polyester resin foam sheets can be thermoformed into heat-resistant food containers which can be used in dual ovenable applications. However, the method disclosed therein has disadvantages in that since not only the melt viscosity is low when a polycarbonate is merely mixed, but carbon dioxide liberated from the polycarbonate is used as a blowing agent, expansion ratio is low, heat resistance is poor and the container cannot bring out bare hands when the articles are used as containers for heating or cooking foods in microwave ovens.
The present inventors have found thermoplastic polyester resin foam sheets having a high expansion ratio, good heat insulating properties and excellent thermofoamability in the above-described study.